Conventionally, quilting have been performed with a sewing machine by stitching a three-layered fabric having materials such as cotton, feather, or urethane foam stuffed between the top cloth and the bottom cloth while forming decorative patterns on the top cloth of the fabric. The user is allowed to freely move the three-layered quilting fabric in given directions to enjoy quilting by combining various stitches such as straight stitches and curved stitches while the sewing machine is in operation.
For example, JP 2002-292175 A (hereinafter referred to as Reference 1) describes a sewing machine provided with a downwardly-oriented sensor mounted at a head of a sewing machine arm. Some of the images captured by the image sensor are inputted to a microcomputer provided in the sewing machine during the sewing operation and the inputted images are take in as static images at predetermined small time intervals. Then, amount of movement of the workpiece cloth is calculated by a first interrupt process. Then, a second interrupt process obtains speed of needle-bar movement, in other words, rotational speed of a sewing machine motor based on a preset “pitch-width” and the calculated amount of workpiece cloth movement. The sewing machine executes a sewing speed control so that the rotational speed of the sewing machine motor is altered from time to time to control the stitch pitch during manual feed so that spacing between the stitches is constant.
The sewing machine described in Reference 1 works fine when patterns can be sewn by effortlessly moving the workpiece such as quilting fabric manually by the user. However, when sewing patterns require dynamic manual movement of the workpiece, the workpiece need to be fed at lower speed. In such case, the needle bar is also moved at lower speed, consequently increasing the time period in which the workpiece is anchored in place by the sewing needle penetrating through it, which in turn reduces the time available for moving the workpiece.
When the user tries to force the movement of the workpiece with the sewing needle penetrating through it, the workpiece is pulled away from the sewing needle. Thus, once the sewing needle is lifted out of the workpiece, the workpiece becomes displaced by the sudden release of tension, resulting in displacement of stitches. The sudden movement of workpiece also causes sudden acceleration of needle-bar movement. When the user brings the workpiece movement to a sudden stop in response to the sudden workpiece movement and acceleration of the needle bar-movement, this time, needle-bar movement is suddenly decelerated.
Thus, when sewing patterns that require dynamic workpiece movement or when the user is a beginner at sewing machine operation, it is desirable to execute the sewing operation under low-speed needle-bar movement, in other words, under low-speed rotation of the sewing machine motor. However, when executing sewing operations at low speed with sewing machines that control sewing speed as described above, it is difficult to sew desired patterns in steady rhythm without stitch displacement.